Texture with transparency becomes black
Let's start with the very basics.
What you call &quot;transparent&quot; is an Alpha Channel at work.
The image you are creating has tree color channels (RGB) and a separate Alpha channel, that is used as a Mask to determine how the image is combined with others.

This is what the Alpha Channel looks like:

The alpha channel is nothing magic, it is just black and white image where the opacity is set by the white element (a value of $1$) and &quot;transparency&quot; by the black (a value of $0$), any gray elements would be &quot;semi-transparent&quot;.
In blender RGB information and Alpha channels are accessed separately, in other words, you must plug the Alpha channel to something to determine what gets the RGB information and what doesn't.
The most basic setup is like this.

The color of the shader is provided by a mix RGB node, that is using the RGB info from the texture (color) and the alpha channel separately, as factor for the mix with a yellow color.
The alpha channel is used to determine what parts of the texture get what color. What is black on the alpha channel will be yellow, what is white in the alpha channel will have the color (RGB) information of the image texture used.
Or you can use the alpha channel to mix different shaders:


UV mapping.
By default blender creates a UV map when you create a cube, so there is a UV map already present in all of the other faces.

The simplest way to deal with would be creating a new material and assign it to that face, but if you want to keep using a single material for the whole thing, you can edit the UV map so that all of the other faces' UV map out is of the picture, and make sure that the extrapolation for the texture used in the material is set to clip, that way anything out of the 0-1 UV space will be ignored.

If the image contains an alpha channel usually blender will pick it up, but in case it doesn't you can enable it manually.